Anti-lock hydraulic brake system

ABSTRACT

A hydraulic anti-lock brake system is disclosed in which the pressure in the wheel brakes can be regulated by means of an inlet valve and an outlet valve in response to the wheel rotational pattern. The wheel brake is supplied with pressure fluid out of the pumps through the inlet valve, and pressurized fluid is taken therefrom through the outlet valve. A differential pressure limiter (18) in front of the inlet valve (15) limits the difference in pressure allowed to build up across the inlet valve by closing a valve until a sufficient pressure build up occurs on the wheel brake. This considerably decreases the noise developed during valve operation. A pressure relief valve (30) insures that an evenly high pressure build-up takes place when the inlet valve (15) is closed by opening when a predetermined pressure difference develops across the inlet valve to allow a higher wheel brake pressure to equalize with the pressure upstream from the inlet valve.

BACKGROUND OF THE INVENTION

The present invention relates to an anti-lock hydraulic brake systemwith at least one wheel brake and an auxiliary pressure source, whereinan inlet valve is inserted into the line between the auxiliary pressuresource and the wheel brake and the wheel brake is connected to a returnline via an outlet valve, wherein a differential pressure limiter isdisposed within the line between the auxiliary pressure source and theinlet valve, the closure member of which is acted upon by both the forceof a valve spring and the pressure in the wheel brake in a manneropening the line.

A brake system of this type is known from German published patentapplication 38 42 699.1. The latter describes a dual-circuit brakesystem with a tandem master cylinder and a pump for each brake circuit.The wheel brake cylinders each communicate via a respective inlet valvewith the pressure line leading to the pump and, also with the brake lineleading to the master brake cylinder. Each brake cylinder communicatesvia a respective outlet valve with the return line to the supplyreservoir. Each inlet valve is normally open, while the outlet valvesare normally closed.

The rotational behavior of the wheels is constantly monitored during abraking operation. In the event of an imminent locked condition of anyone of the wheels, the drive of the pump is activated, and the inlet andoutlet valves of the imminently locking wheel controlled such as todevelop an amount of pressure in the wheel brakes which correlates withan optimal slip value of the wheel. To decrease the pressure, the outletvalve is opened and the inlet valve closed. Both valves are closed toretain the pressure, whilst for the pressure increase the inlet valve isopen and the outlet valve is closed.

The inlet and the outlet valves are electromagnetically operated valveshaving a closure member which is seated on a valve seat.

Starting from a brake fluid pressure which is in excess of the pressurewhich corresponds to the force of the valve spring, a differentialpressure limiter between the auxiliary pressure source and the inletvalve causes a defined constant pressure gradient to prevail on theinlet valve which is determined by the valve spring. This pressuregradient can be adjusted to 25 bar for example, and hence isconsiderably less than the pressure gradient which usually occurs whenan emergency stop anti-lock controlled braking operation is performed bythe brake system.

However, it has been discovered that dynamic actions in the return lineand in the differential pressure limiter can cause the pressure in frontof the inlet valve to build up to a level higher than what is desired.

Hence the present invention has for its object to develop measuresensuring that the pressure gradient at the inlet valve remains constantunder all circumstances.

SUMMARY OF THE INVENTION

This object is achieved in that a pressure relief valve opening towardsthe wheel brake is connected in parallel to the inlet valve.

To realise the circuitry in a compact unit which conforms to all demandsmade on the stability of the control operation in the differentialpressure limiter, the differential pressure limiter has a housing whichincorporates three chambers, a control chamber, an inlet chamber, and anoutlet chamber. A valve member is included comprised of a stepped pistonfurnished with an annular groove. The housing is provided with at leastone port terminating into the annular groove of the stepped piston, anda piston part adjacent to the annular groove is slidable in the way of avalve slide to close the port.

The differential pressure limiting valve is thus designed as a valveslide which, in addition, permits the pressure relief valve to beincorporated in the piston of the valve slide.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a hydraulic circuit diagram of the brake system including amaster cylinder shown in cross-section.

FIG. 2 is a cross sectional view of a first embodiment of a differentialpressure limiter included in the brake system of FIG. 1.

FIG. 3 is a cross sectional view of a second embodiment of thedifferential pressure limiter included in the brake system of FIG. 1.

DETAILED DESCRIPTION

The hydraulic brake system is composed of a master brake cylinder 1connected to a pneumatic booster 2. The pneumatic booster 2 is in turnoperated by a brake pedal 3. A floating piston 5 and a push rod piston 6are sealing guided in the longitudinal bore 4 of the housing of themaster brake cylinder 1. They define two working chambers 7 and 8 in thebore 4 of the master brake cylinder 1. Both the push rod piston 6 andthe floating piston 5 comprise a central valve 9 whose closure member10, in the initial position of the piston, is kept spaced from the valveseat by means of a tappet which is placed on a stop on the housing.Thereby, a connection is established between the working chambers 7 and8 and the supply reservoir 11. When the driver depresses the pedal 3,the push rod piston 6 will be shifted into the master brake cylinderassisted by the pneumatic booster 2, in consequence whereof the centralvalve 9 closes and pressure fluid displaces out of the working chamber 8into the connected brake circuit As a result, the floating displaces outof the working chamber 7 also. Both working chambers are incommunication with the wheel brakes 32 of the vehicle via a brake line12. The wheel brakes are referred to by a combination of letters, thefollowing abbreviations being chosen: V for front, H for rear, L forleft and R for right.

While only one brake circuit is shown, the non-illustrated circuit is ofcorresponding design.

The brake system also includes a double pump 13 which is driven by amotor M. The pumps 13 deliver fluid out of the supply reservoir 11 viaeach pressure line 14 into the brake lines 12. Inserted into each branchof the brake line 12 to the wheel brakes, is an inlet valve 15 which isoperated electromagnetically and is open in its initial position.

The wheel brakes also communicate, via a return line 16, with the supplyreservoir 11, with each outlet valve 17 being inserted into the returnline 16. Each outlet valve 17 is likewise operated electromagneticallyto be opened, and is closed in its de-energized state.

Upstream of each of the inlet valves 15, between the inlet valve 15 andthe pump 13 and the master brake cylinder 1, is a differential pressurelimiter 18 having a control line 19 connected directly to the respectivewheel brake.

A non-return valve 21 closing towards the wheel brake is inserted into aby-pass line 20 in parallel to the inlet valve 15 and the differentialpressure limiter 18.

Branching off from the control line 19 is a pressure relief valve 30which connects the control line 19 with the brake line portion betweenthe differential pressure limiter 18 and the inlet valve 15. Thepressure relief valve 30 opens towards the control line.

A restrictor 31 which is normally integrated in the inlet valve isarranged directly beneath the control line's junction into the brakeline for reasons of control characteristics.

The differential pressure limiter 18 is illustrated in more detail inFIG. 2. It is composed of a housing with three chambers arranged side byside. On the right hand in the illustration, there is the outlet chamber37 which communicates via the inlet valve 15 with the wheel brake.

On the left hand of the outlet chamber 37, there is the inlet chamber 38which communicates with the pressure line 14 and the brake line 12,respectively. The chambers 37 and 38 are isolated from each other by awall 22, a central hole in the wall 22 forming the valve passage. To theleft of the inlet chamber 38 is the control chamber 23 which, via thecontrol line 19, is directly communicating with the wheel brake. A valveshaft 25 extends sealingly through the wall 24 between inlet chamber 38and control chamber 23, which valve shaft projects through the centralhole in the wall 22 into the outlet chamber 37. There it passes overinto a valve closure member 26 of conical shape.

A valve spring 27 is arranged in the control chamber 23 and bearsagainst the housing bottom, on one end, and on the valve shaft 25, onthe other end. The valve spring 27 urges the shaft 25 against a stop 39,while in the basic position which is defined by the stop 39 the valvemember 26 is held at a distance from the valve seat 28. In this eventthere is a free passage between the inlet chamber 38 and the outletchamber 39.

The system operates in the following fashion:

In the initial position all parts assume their illustrated position, andthe valves assume their illustrated switch positions. Operation of thepedal 3 causes pressure fluid to displace out of the working chambers 7and 8 into the brake lines 12 via the open valves 15 to the wheelbrakes, whereby pressure develops in the brake circuits which results inthe brakes being applied and the vehicle being decelerated. Thedifferential pressure limiter remains open, since the same amount ofpressure prevails both in the outlet chamber 37 and in the controlchamber 23. Hence the valve shaft 25 is force balanced with respect tothe acting pressures so that the spring 27 is capable of keeping thevalve 26, 28 open. The pressure relief valve 30 is likewise closed.

Once the pressure in the brake circuits is increased by the pedal forceto the extent that the wheels tend to lock, the anti-lock mode willcommence. The motor M of the pumps 13 is switched on and the inlet valve15 closes. Now the pressure in the pressure line and, respectively, inthe brake line can be increased without an increase in pressure in thewheel brakes. Since now the pressure in the outlet chamber 37 is inexcess of that in the control chamber 23, a force acts upon the shaft 25which works in opposition to the spring force and displaces the shaft tothe left as viewed in FIG. 2. This causes the valve member 26 to sit onthe valve seat 28, and the connection is closed between the inletchamber 38 and the outlet chamber 37. The bias of the spring 27 may beadjusted, e.g., such that the valve will be closed in the presence of adifference in pressure of, e.g., 25 bar between the outlet chamber 37and the control chamber 23. The pressure in the wheel brakes can bedecreased still more by opening the outlet valve 17. If now, in order tocause pressure re-increase in the wheel brakes, the inlet valve 15 isopened, first the pressures in the outlet chamber 37 and the controlchamber 23 will adapt again so that the valve passage of thedifferential pressure limiter will be opened and pressure fluid can flowout of the pump 13 into the wheel brakes as soon as the pressuredifference amounts to 25 bar again.

It is essential that a defined constant pressure gradient is alwaysadjusted by the differential pressure limiter 18 on the inlet valve 15,provided that the brake fluid pressure exceeds the pressure whichcorresponds to the force of the valve spring 27.

The pressure relief valve 30 prevents dynamic actions from causing thepressure in front of the inlet valve 15 to become higher than that inthe wheel brake by more than roughly 25 bar. That is should thissituation occur, the pressure relief valve 30 will open and effect acorresponding pressure compensation. It will be ensured thereby that anevenly high pressure build-up takes place in front of the inlet valve 15when the inlet valve 15 is closed.

The illustration according to FIG. 3 corresponds to that one of FIG. 1as regards the valve switching arrangement. Like reference numerals havebeen used hence. In addition, the following applies:

The brake line portion 46 between the differential pressure limiter 18and the inlet valve 15 is connected with the control chamber 23 by meansof a pressure relief valve 30'. The pressure relief valve 30' openstowards the control chamber 23'.

The restrictor 31 which is normally integrated in the inlet valve isinterposed in the brake line directly downstream of the control chamberfor reasons relating to the improvement of the control quality.

The differential pressure limiter 18' is accommodated in a housing 40which includes a stepped blind-end bore 41. The smaller step is disposedfarther inwardly, while the larger step opens to the outside and,respectively, is closed by a plug 42. A stepped piston 43 is guided inthe blind-end bore and separates the outlet chamber 37 with itssmall-diameter portion in the smaller step of the blind-end bore, whilewith its larger-diameter portion it confines the control chamber 23' inthe area of the larger step of the blind-end bore.

The control chamber 37' is in communication with the inlet valve 15 viaa housing port 45 and a line 46. The line portion 46 is part of thebrake line 12. The control chamber 23' is provided with a first port 48which connects to the other side of the inlet valve and with a port 49connecting to the wheel brake. A restrictor 31 is inserted between theport 49 and the wheel brake 4.

The control chamber 47 also accommodates the control spring 50 which issupported on the stepped piston 43, on one end, and on the plug 42, onthe other end. The control spring 50 keeps the stepped piston 43 on astop 51 which is formed by the step between the small and the largerportion of the stepped bore in this embodiment.

In the area of its small diameter, the stepped piston 43 is furnishedwith a circumferential groove 52 acting as inlet chamber 38. Hence,there is a port 58 which connects to the master brake cylinder. Anotherport 59 is also connected to the master brake cylinder and is opposed tothe port 58 so that uniform pressurization takes place. A longitudinalbore 53 extends through the length of the stepped piston 43 and leadsfrom the one end face at the outlet chamber 37' to the other end face atthe control chamber 23'. A transverse bore 54 extends vertically to thelongitudinal bore 53 and terminates on both sides into the groove 52 andcrosses the longitudinal bore 53 in the axis of the piston 43. This waythere is a connection between the groove 52 and the outlet chamber 37'.

The part of the longitudinal bore 53 leading to the control chamber 23'is provided with the pressure relief valve 30'. To this end, thelongitudinal bore 53 includes a step which is designed as valve seat 55.Abutting on this valve seat 55 is a valve ball 56 which is held by meansof a valve spring 57. The valve spring 57 is supported on an annularinsert member 62 which is inserted into the one end face of the steppedpiston 43. On the side close to the inlet chamber 37', the groove 52 isconfined by a piston part 60 having a control edge 61. The control edge61 is disposed directly in front of the ports 58 and 59 in theillustrated initial position of the stepped piston 43. Hence when thestepped piston 43 is displaced in opposition to the force of the spring50, the control edge 61 will shift over the ports 58 and 59 so that thepiston part 60 closes the ports and isolates the groove 52 from themaster brake cylinder. The brake line 12 is interrupted thereby. Thecontrol edge 61 in conjunction with the ports 58, 59 thus acts as avalve.

Furthermore, the pressure difference between the outlet chamber 37' andthe control chamber 23' is adjusted to a defined valve by the pressurerelief valve 30.

The system operates in the following manner:

In the initial position all parts assume their illustrated position, andthe valves assume their illustrated switch positions. Operation of thepedal 3 causes pressure fluid to displace out of the master brakecylinder 1 into the brake lines 12 via the open valves 15 to the wheelbrakes, whereby pressure develops in the brake circuits which results inthe brakes being applied and the vehicle being decelerated. Thedifferential pressure limiter remains open, since the same amount ofpressure prevails both in the outlet chamber 37' and in the controlchamber 23'. Hence the stepped piston 43 is balanced in force withrespect to the acting pressures so that the spring 50 is able to keepthe valve elements 58, 59, 60 of the differential pressure limiter 18open. The pressure relief valve 30' is likewise closed.

Once the pressure in the brake circuits is increased sufficiently by thepedal force that the wheels tend to lock, the anti-lock mode willcommence. The drive of the pump 13 is switched on and the inlet valve 15closes. Now the pressure in the pressure line and brake line can beincreased without the pressure in the wheel brakes increasing. Since nowthe pressure in the outlet chamber 37' is in excess of that in thecontrol chamber 23', a force acts upon the stepped piston 43 which worksin opposition to the spring force and displaces the stepped piston tothe left, according to the illustration of FIG. 3. This causes thepiston part 60 to shift in front of the ports 58, 59, whereby theconnection is closed between the master brake cylinder 1 and the outletchamber 37'. The bias of the spring 50 may be adjusted e.g., such thatthe valve will be closed in the presence of a difference in pressure ofe.g., 25 bar between the outlet chamber 37' and the control chamber 23'.The pressure in the wheel brakes can be decreased still more by openingthe outlet valve 17. If now, in order to cause pressure re-increase inthe wheel brakes, the inlet valve 15 is opened, first the pressures inthe outlet chamber 37' and the control chamber 23' will adapt to oneanother again so that the valve elements 58, 59, 60 of the differentialpressure limiter 18' will be opened by the control valve spring 50 andpressure fluid can flow out of the pump 13 into the wheel brakes.

It is essential that a defined constant pressure gradient is alwaysadjusted by the differential pressure limiter on the inlet valve 15,provided that the brake fluid pressure exceeds the pressure whichcorresponds to the force of the valve spring 50.

The pressure relief valve 30' prevents dynamic actions from causing thepressure in front of the inlet valve becoming higher that in the wheelbrake by more than roughly 25 bar. Should this situation occur, thepressure relief valve 30 will open and effect a corresponding pressurecompensation. It will be ensured thereby that an evenly high pressurebuild-up takes place in front of the inlet valve 15 when the inlet valve15 is closed.

The valve slide action safeguards a sufficiently precise regulation ofthe differential pressure. It is possible at the same time that adefined amount of leakage is adjusted on the piston part 60 so that adefined quantity of pressure fluid per time unit is flowing. When thisquantity ensures roughly the fluid demand for the control, there is noneed to switch the inlet valve 15, in consequence whereof the noisedeveloping is diminished further.

I claim:
 1. An anti-lock hydraulic brake system with at least one wheel brake and an auxiliary pressure source, a pressure line between said auxiliary pressure source and said wheel brake, an inlet valve in said pressure line, a return line, an outlet valve, said wheel brake connected to said return line via said outlet valve, a differential pressure limiter means disposed within said pressure line between said auxiliary pressure source and said inlet valve, said differential pressure limiter means including a closure member, a valve spring acting on said closure member, a control line extending in parallel to said pressure line and connected thereto downstream of said inlet valve causing said closure member to also be acted upon by the fluid pressure in said wheel brake to cause opening of said pressure line, wherein a pressure relief valve means opening towards the wheel brake is connected in parallel to said inlet valve, said pressure relief valve means opening communication past said inlet valve whenever a predetermined high pressure develops across said inlet valve.
 2. A brake system as claimed in claim 1, wherein said brake system includes a pedal operated master brake cylinder having a working chamber connected by way of a brake line connected to said pressure line between said auxiliary pressure source and said differential pressure limiter.
 3. A brake system as claimed in claim 1, wherein said return line terminates into a supply reservoir and said auxiliary pressure source comprises at least one pump supplying fluid out of said supply reservoir.
 4. A brake system as claimed in claim 1, wherein said differential pressure limiter means includes a housing having partitions defining three chambers, a control chamber, an inlet chamber, and an outlet chamber, a passage between said inlet chamber and said outlet chamber, a valve member and a valve seat, said valve member and said valve seat arranged to be movable between positions opening and closing said passage, said valve member having a valve shaft extending through one of said partitions and into said control chamber, said valve shaft sealingly guided through said one partition between said inlet chamber and said control chamber.
 5. A brake system as claimed in claim 4, wherein said control chamber is connected to said wheel brake, said inlet chamber is connected to said auxiliary pressure source, and said outlet chamber is connected via said inlet valve with said wheel brake.
 6. A brake system as claimed in claim 4, wherein said control chamber houses said valve spring, said valve spring having one end engaged on said housing, and also having another end engaging said valve shaft, said valve spring urging said valve member away from said valve seat.
 7. A brake system as claimed in claim 1, wherein a restrictor is arranged in said pressure line between the connection of said control line to said pressure line downstream of said inlet valve and said wheel brake.
 8. A brake system as claimed in claim 1, wherein said differential pressure limiter means includes a housing defining three chambers, a control chamber, an inlet chamber, and an outlet chamber, in that said closure member is a stepped piston, an annular groove, said housing provided with at least one port terminating into said annular groove, and in that a piston part adjacent to said annular groove is slidable as a valve slide to close said port.
 9. A brake system as claimed in claim 8, wherein said control chamber is connected to said wheel brake, said inlet chamber via said annular groove is connected to said auxiliary pressure source and said outlet chamber is connected via said inlet valve to said wheel brake.
 10. A brake system as claimed in claim 8, wherein said stepped piston has a longitudinal bore and a transverse bore said longitudinal bore having respective portions connecting said groove with said outlet chamber and said control chamber, said pressure relief valve having a valve member and a valve seat, said valve member being inserted into said portion of said longitudinal bore connected to said control chamber, and said valve seat carried by said stepped piston.
 11. A brake system as claimed 8, wherein said piston part has a slightly smaller diameter than the corresponding bore portion so that leakage flows past said piston port between said annular groove and said outlet chamber. 